An electric power steering apparatus (EPS) which is equipped with an electronic control unit and provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control. The motor is driven and controlled by the inverter which is constituted by FET bridges as semiconductor switching devices.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13 as a power supply, and an ignition key (IG) signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist-control on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the calculated current command value. The steering angle θ can be obtained from a rotational sensor connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a non-CAN 41 is also possible to connect to the control unit 30, and the non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises an MCU (including a CPU, an MPU and the like), and general functions performed by programs within the MCU are, for example, shown in FIG. 2. Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after the addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result I(=Irefm−Im) in the subtracting section 32B is proportional-integral-controlled (PI-controlled) at a PI-control section 35. The voltage control command value Vref obtained by the PI-control and a modulation signal (a carrier) CF are inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The addition result is further added with a convergence control value 341 at an adding section 345. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the control characteristics.
In a case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 36 and the inverter 37 have a configuration as shown in FIG. 3, and the PWM-control section 36 comprises a duty calculating section 36A that is within the MCU and calculates duty signals D1 to D6 which are used in a three-phase PWM-control by using the voltage control command value Vref in accordance with a predetermined equation, and a gate driving section 36B that drives the gates of the FETs as the semiconductor switching devices by means of the duty signals D1 to D6 and turns the gates on or off with compensating a dead time. The modulation signal (the carrier) CF is inputted into the duty calculating section 36A, and the duty calculating section 36A calculates the duty signals D1 to D6 of the PWM by synchronized to the modulation signal CF.
The inverter 37 is configured to the three-phase bridges of the upper stage FET 1 to FET 3 and the lower stage FET 4 to FET 6. The gate driving section 36B turns the FET 1 to the FET 6 on or off by means of the duty signals D1 to D6 of the PWM respectively, so that the motor 20 is driven. The FET 1 to the FET 6 are the FET with a back flow preventing parasitic diode.
A motor opening switch 23 is interposed between the inverter 37 and the motor 20 in order to interrupt a current supply when the assist-control is stopped and the like. The motor opening switch 23 comprises the FETs with the parasitic diode disposed to respective phases.
In such an electric power steering apparatus, conventionally, a system that detects the abnormality (including the failure) of the MCU is disclosed in, for example, an apparatus described in Japanese Unexamined Patent Publication No. 2003-26024 A (Patent Document 1). In Patent Document 1, as shown in FIG. 4, in order to prevent from dangerous operation when the MCU 100A is abnormal, a WDT 110 serving as the abnormal detecting circuit is disposed in the external of the MCU 100A, the WDT 110 outputs a reset signal RS when detecting the abnormality of the MCU 100A, and a reset circuit 120 that arranges a shape of the reset signal RS and resets the MCU 100A, is provided. The electric power from a power supply generating circuit 130 is supplied to the MCU 100A. The steering torque Th, the vehicle speed Vel, the steering angle θ and the like are inputted into the MCU 100A. The MCU 100A drives and controls the motor 20 by using the calculated duty signals D1 to D6 of the PWM for driving the FETs through the gate driving section 36B and the inverter 37.
The external WDT 110 inputs an abnormal detecting signal from the MCU 100A, outputs the reset signal RS in a case of detecting the abnormal signal, resets the MCU 100A through the reset circuit 120, and stops the MCU 100A and the system, so that the safety is secured.